Managing truncation of files of file systems

ABSTRACT

A method is used in managing truncation of files of file systems. A request is received to delete a portion of a file of a file system. A replica of the file is created. The replica represents a state of the file at a particular prior point in time. The replica shares a set of file system blocks of the file with the file. The portion of the file is deleted by updating metadata of the file. The replica of the file is asynchronously deleted in background by de-allocating the set of file system blocks.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to co-pending U.S. patent application Ser.No. 14/870,556 entitled MANAGING TRUNCATION OF FILES OF FILE SYSTEMSfiled on Sep. 30, 2015, which is incorporated herein by reference.

BACKGROUND Technical Field

This application relates to managing truncation of files of filesystems.

Description of Related Art

Computer systems may include different resources used by one or morehost processors. Resources and host processors in a computer system maybe interconnected by one or more communication connections. Theseresources may include, for example, data storage devices such as thoseincluded in the data storage systems manufactured by EMC Corporation.These data storage systems may be coupled to one or more servers or hostprocessors and provide storage services to each host processor. Multipledata storage systems from one or more different vendors may be connectedand may provide common data storage for one or more host processors in acomputer system.

A host processor may perform a variety of data processing tasks andoperations using the data storage system. For example, a host processormay perform basic system I/O operations in connection with datarequests, such as data read and write operations.

Host processor systems may store and retrieve data using a storagedevice containing a plurality of host interface units, disk drives, anddisk interface units. The host systems access the storage device througha plurality of channels provided therewith. Host systems provide dataand access control information through the channels to the storagedevice and the storage device provides data to the host systems alsothrough the channels. The host systems do not address the disk drives ofthe storage device directly, but rather, access what appears to the hostsystems as a plurality of logical disk units. The logical disk units mayor may not correspond to the actual disk drives. Allowing multiple hostsystems to access the single storage device unit allows the host systemsto share data in the device. In order to facilitate sharing of the dataon the device, additional software on the data storage systems may alsobe used.

Additionally, the need for high performance, high capacity informationtechnology systems are driven by several factors. In many industries,critical information technology applications require outstanding levelsof service. At the same time, the world is experiencing an informationexplosion as more and more users demand timely access to a huge andsteadily growing mass of data including high quality multimedia content.The users also demand that information technology solutions protect dataand perform under harsh conditions with minimal data loss and minimumdata unavailability. Computing systems of all types are not onlyaccommodating more data but are also becoming more and moreinterconnected, raising the amounts of data exchanged at a geometricrate.

To address this demand, modern data storage systems (“storage systems”)are put to a variety of commercial uses. For example, they are coupledwith host systems to store data for purposes of product development, andlarge storage systems are used by financial institutions to storecritical data in large databases.

In data storage systems where high-availability is a necessity, systemadministrators are constantly faced with the challenges of preservingdata integrity and ensuring availability of critical system components.One critical system component in any computer processing system is itsfile system. File systems include software programs and data structuresthat define the use of underlying data storage devices. File systems areresponsible for organizing disk storage into files and directories andkeeping track of which part of disk storage belong to which file andwhich are not being used.

An operating system, executing on a data storage system such as a fileserver, controls the allocation of a memory of the data storage systemto host systems or clients connected to the data storage system.Allocation is generally performed at a page granularity, where a page isa selected number of contiguous blocks. The particular size of a page istypically a function of an operating system, the page size may be 8kilobytes (KB).

To the operating system of a data storage system, a file system is acollection of file system blocks of a specific size. For example, thesize of a file system block may be 8 kilobytes (KB). As the data storagesystem is initialized, some of the pages are reserved for use by theoperating system, some pages are designated as ‘free’ for allocation toother applications, and a large chunk of pages are reserved to provide abuffer cache (also referred to as “buffer cache pool”). The buffer cachetemporarily stores pages in a volatile memory of a data storage systemthat are also stored in an attached disk device to increase applicationperformance.

File systems typically include metadata describing attributes of a filesystem and data from a user of the file system. A file system contains arange of file system blocks that store metadata and data. A user of afile system access the file system using a logical address (a relativeoffset in a file) and the file system converts the logical address to aphysical address of a disk storage that stores the file system. Further,a user of a data storage system creates one or more files in a filesystem. Every file includes an index node (also referred to simply as“inode”) that contains the metadata (such as permissions, ownerships,timestamps) about that file. The contents of a file are stored in acollection of data blocks. An inode of a file defines an address mapthat converts a logical address of the file to a physical address of thefile. Further, in order to create the address map, the inode includesdirect data block pointers and indirect block pointers. A data blockpointer points to a data block of a file system that contains user data.An indirect block pointer points to an indirect block that contains anarray of block pointers (to either other indirect blocks or to datablocks). There may be many levels of indirect blocks arranged in ahierarchy depending upon the size of a file where each level of indirectblocks includes pointers to indirect blocks at the next lower level.

A file may be replicated by using a snapshot copy facility that createsone or more replicas (also referred to as “snapshot copies”) of thefile. A replica of a file is a point-in-time copy of the file. Further,each replica of a file is represented by a version file that includes aninheritance mechanism enabling metadata (e.g., indirect blocks) and data(e.g., direct data blocks) of the file to be shared across one or moreversions of the file.

Although existing various methods provide reasonable means of writingdata to file systems stored to a persistent storage, providing access todata of file systems and creating a replica of file systems, they alsocome with a number of challenges, especially when efficiently truncatinga file of a file system. It may be difficult or impossible for theconventional file system facility to efficiently truncate a file of afile system.

SUMMARY OF THE INVENTION

A method is used in managing truncation of files of file systems. Arequest is received to delete a portion of a file of a file system. Areplica of the file is created. The replica represents a state of thefile at a particular prior point in time. The replica shares a set offile system blocks of the file with the file. The portion of the file isdeleted by updating metadata of the file. The replica of the file isasynchronously deleted in background by de-allocating the set of filesystem blocks.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the present invention will become moreapparent from the following detailed description of exemplaryembodiments thereof taken in conjunction with the accompanying drawingsin which:

FIGS. 1-2 are examples of an embodiment of a computer system that mayutilize the techniques described herein;

FIG. 3 is an example illustrating storage device layout;

FIGS. 4-8 are diagrams illustrating in more detail components that maybe used in connection with techniques herein; and

FIG. 9 is a flow diagram illustrating processes that may be used inconnection with techniques herein.

DETAILED DESCRIPTION OF EMBODIMENT(S)

Described below is a technique for use in managing truncation of filesof file systems, which technique may be used to provide, among otherthings, receiving a request to delete a portion of a file of a filesystem, creating a replica of the file, where the replica represents astate of the file at a particular prior point in time, where the replicashares a set of file system blocks of the file with the file, deletingthe portion of the file by updating metadata of the file, andasynchronously deleting the replica of the file in background byde-allocating the set of file system blocks.

Generally, a storage extent is a logical contiguous area of storagereserved for a user requesting the storage space. A storage extent mayinclude a set of disks having different RAID levels. A disk may be aphysical disk within the storage system. A LUN may be a logical unitnumber which is an identifier for a logical unit representing a portionof disk storage. Each slice of data may have a mapping to the locationof the physical drive where it starts and ends. A LUN presented to ahost system may be organized as a file system by a file system mappinglogic of a storage system.

A file is uniquely identified by a file system identification number.Each data block of a file is referenced by a logical block number and/orfile system block number. A logical block number of a file refers to adata block by relative position of the data block inside the file. Afile system block number of a file refers to a data block by relativeposition of the data block on a physical disk device on which the fileis stored. A file system block number for a data block is computed basedon a file offset and the size of the data block. Further, an inode of afile includes metadata that provides a mapping to convert a file systemblock number of a data block to its corresponding logical block number.For example, in case of a data block size of 4 kilobytes (KB), if a fileoffset value is smaller than 4096 bytes, the file offset corresponds tothe first data block of the file, which has file block number 0.Further, for example, if a file offset value is equal to or greater than4096 bytes and less than 8192 bytes, the file offset corresponds to thesecond data block of the file, which has file block number 1.

Generally, each file system data block of a file is associated with arespective mapping pointer. A mapping pointer of a file system blockpoints to the file system block and includes metadata information forthe file system block. A file system block associated with a mappingpointer may be a data block or an indirect block which in turn points toother data blocks or indirect blocks. A mapping pointer includesinformation that help map a logical offset of a file system block to acorresponding physical block address of the file system block.

Further, a mapping pointer of a file system block includes metadatainformation for the file system block such as a weight that indicates adelegated reference count for the mapping pointer. The delegatedreference count is used by a snapshot copy facility when a replica of afile is created. Mapping pointers of the inode of the file are copiedand included in the inode of the replica of the file. Mapping pointersof the inode may include mapping pointers pointing to direct data blocksand mapping pointers pointing to indirect blocks. The delegatedreference count values stored in the mapping pointers of the file andthe replica of the file are updated to indicate that the file and thereplica of the file share data blocks of the file.

The delegated reference counting mechanism is described in U.S. Pat. No.8,032,498 for “Delegated reference count base file versioning” issuedOct. 4, 2011, which is incorporated herein by reference.

Further, the delegated reference counting mechanism is also used by adeduplication facility for performing deduplication on a set ofidentical data blocks by sharing the set of identical data blocks andkeeping a single copy of data block such that other identical datablocks point to the single copy of the data block.

Thus, a delegated reference count is a way of maintaining blockownership information for indicating whether or not each indirect blockor data block of a file is shared with another version of the file oranother identical data block. Further, as introduced above, files areorganized as a hierarchy of file system blocks including inodes,indirect blocks, and data blocks. The hierarchy of file system blocksincludes a parent-child block relationship between a parent object thatpoints to a child object. For example, if the mapping pointer of theinode of a file points to a data block, the association between themapping pointer of the inode and the data block may be viewed as aparent-child block relationship. Similarly, for example, if the mappingpointer of an indirect block of a file points to a data block, theassociation between the mapping pointer of the indirect block and thedata block may be viewed as a parent-child block relationship. Blockownership information is maintained by storing respective referencecounts for the file system indirect blocks and file system data blocksin the file system block hierarchy, and by storing respective delegatedreference counts for the parent-child block relationships in the filesystem block hierarchy. For each parent-child block relationship, acomparison of the respective delegated reference count for theparent-child relationship to the reference count for the child blockindicates whether or not the child block is either shared among parentblocks or has a single, exclusive parent block. For example, if therespective delegated reference count is equal to the respectivereference count, then the child block is not shared, and the parentblock is the exclusive parent of the child block. Otherwise, if therespective delegated reference count is not equal to the respectivereference count, then the child block is shared among parent blocks.

Further, when a sharing relationship of a file system block is broken,the reference count in the per-block metadata of the file system blockis decremented by the delegated reference count associated with mappingpointer of the file system block.

A snapshot (also referred to herein as “replica”, “checkpoint”, and“snap”) is a point-in-time copy of data (e.g., a production file).Generally, storage applications use snapshots to protect production dataand ensure consistency of the production data. Generally, snapshots ofdata are created at a regular time interval (e.g., 10 minutes, 1 hour).Each snapshot of data has a unique identification. When an old snapshotis deleted, storage space allocated to the old snapshot is reclaimed andmetadata associated with the old snapshot is updated appropriately. Aset of replicas of a file (also referred to herein as “working file” or“primary file”) may be logically organized together in a version set. Aversion set indicates a family of snapshot copies.

Typically, a file delete operation deletes a file of a file system in astorage system. When a file is deleted, each file system block of thefile is deleted and marked as a free file system block. Further, a filemay be truncated by deleting a portion of the file. Generally, during afile truncate operation, each file system block that is a part of aportion of the file which is being truncated is deleted. When a filesystem block is deleted, a parent file system block which includes amapping pointer pointing to the file system block is updated to indicatethat the mapping pointer no longer points to the file system block. Byupdating the mapping pointer in such a way creates a hole such that themapping pointer is marked as unused indicating that the mapping pointerno longer points to any file system block.

Generally, upon receiving a request to delete a file or a portion of thefile (also referred to herein as “truncating a file”), a file systemhierarchy of the entire file or the portion of the file is iterated tofree each file system block of the file system hierarchy of the entirefile or the portion of the file. An indirect block at the lowest levelof a file system hierarchy of a file is known as a leaf indirect block.Thus, each leaf indirect block of the file system hierarchy of a file ora portion of the file is processed for deleting data blocks pointed toby each leaf indirect block. It should be noted that a file delete orfile truncate operation may either be executed on a primary file or anyreplica of the primary file.

Conventionally, a file truncate operation to truncate a file by deletinga portion of the file traverses a file system block hierarchy associatedwith the portion of the file in order to delete each file system blockincluded in the portion of the file, de-allocates each file system blockincluded in the portion of the file identified for deletion, and createsa metadata transaction entry for each file system block deleted by thefile truncate operation. In such a conventional system, if a deleteoperation or a truncate operation is performed on a large number offiles, for example, hundreds of files with the size of 100 gigabytes(GB), the delete or truncate operations may require a large amount oftime such as hours to delete or truncate the files because the delete ortruncate operations either traverses the entire file system blockhierarchy or a large portion of the file system block hierarchy of eachfile for de-allocating file system blocks and creates a metadatatransaction entry for each file system block being deleted and storesthe metadata transaction entry in a file system transaction log, andlater updates metadata organized on a storage device by flushing thefile system transaction log. Further, in such a conventional system,reading and updating of per-block metadata structures of each filesystem block of a file system hierarchy of each file being truncatedrequires a significant amount of time in case the size of the filesystem block hierarchy is large. Consequently, in such a conventionalsystem, a large number of resources of a storage system are consumedresulting in increase in CPU consumption because a large amount of dataand/or metadata is copied to and from a memory and a large number oflocks for file system and storage system structures are acquired andreleased.

Thus, in such a conventional system, depending on the size of a file,truncate operation can potentially take hours to complete.Conventionally, when a truncate operation is performed synchronously ona file, a lock is acquired on the file for the entire duration of timethe truncate operation is being performed on the file. Consequently, insuch a conventional system, when a lock is acquired on a file forperforming a truncate operation, a storage system is unable to performany other operation on the file requested by clients of the storagesystem. Further, in such a conventional system, if a truncate operationis performed asynchronously on a file, it may be difficult or impossibleto maintain correct locking semantics for processing various truncate,write and read operations being issued concurrently on the file.

Thus, conventionally, truncating a file consumes a large amount ofstorage resources such as CPU and I/O load because a large amount ofmetadata is accessed and updated when truncating the file. Further, insuch a conventional system, a significant amount of time is spentdeleting a file thereby impacting performance of other I/O operationsexecuting concurrently in a storage system. Further, in such aconventional system, if a large number of files are truncated, theamount of time required to delete the large number of files may impactperformance of other facilities executing in such a conventional system.Thus, in such a conventional system, truncating a large number of filesmay consume a large amount of time thereby causing a delay in reclaimingstorage space associated with portions of the files identified fortruncation. Thus, a goal of the current invention is to efficientlytruncate files in order to increase performance of a system byperforming less number of I/O operations and consuming less storageresources (e.g., CPU and cache) of the system.

For example, in such a conventional system, if a storage system receivesfollowing series of request: 1) request to write a data pattern to afile starting from file offset of 0x6429fb to 0x64a564, 2) a truncateoperation to truncate the file to size 0x81f62, and 3) a request towrite data to the file at offset 0x1109ad7 to increase the size of thefile. In such a conventional case, a read request performed at the fileoffset of 0x6450ec may return stale data (e.g., data written by thefirst write instead of empty data (e.g., zeros)) because the truncateoperation indicated by the second request above herein may be performedon the file after the write data request, the first request indicatedabove herein, thereby providing old data to a client due to incorrectsemantics employed by the conventional storage system. Further, in sucha conventional system, when a range lock is acquired for the entireportion of a file identified for truncation or deletion duringperformance of a truncate operation or a delete operation, a concurrentwrite operation pending on the file may timeout while waiting for thetruncate operation to finish as the truncate operation in such aconventional case may require a significant amount of time (e.g., hours)to complete because the truncate operation requires time to free eachdata block included in a file system block hierarchy identified fordeletion by the truncate operation.

By contrast, in at least some implementations in accordance with thetechnique as described herein, the current technique optimizes filetruncate and file delete operations by creating an internal snapshotcopy of a file identified for truncation or deletion and deleting thefile by using delegated reference count mechanism thereby improvingperformance of the file truncate and delete operations and reducingimpact of the file truncate and delete operations on a storage system.In at least one embodiment of the current technique, initially, eachfile system block included in a file are shared between the file and theinternal snapshot copy of the file created upon receiving a truncateoperation. Thus, in at least one embodiment of the current technique, afile can be truncated in a short amount of time because deleting suchshared file system blocks include updating metadata of the file toindicate that the file no longer references the shared file system datablocks instead of having to deallocate each file system block isperformed in a conventional system. Further, in at least one embodimentof the current technique, the internal snapshot copy of a file isdeleted asynchronously in the background to deallocate file systemblocks associated with a truncate operation directed to the file. Itshould be noted that an internal snapshot copy of a file created whenperforming a truncate operation is not visible to a user or host of astorage system but used internally by the storage system to efficientlytruncate the file. Further, when a subsequent truncate operation isreceived for the file and an internal snapshot copy of the file is beingtruncated in background, another snapshot copy of the file is created toperform the subsequent truncation operation on the file.

A snap delete is a process that deletes file system blocks included in afile system hierarchy of a snap of a storage object such as a file byde-allocating the file system blocks. It should be noted that the term“snap delete”, “replica delete”, “file delete”, and “file truncate” maybe used herein interchangeably.

In at least one embodiment of the current technique, distributed weightfor each shared data block included in a portion of file systemhierarchy of a file identified for file truncation or file deletion isupdated to return the weight value of a shared data block to an internalsnapshot copy of the file indicating that the file no longer referencesto the shared data block. Thus, in at least one embodiment of thecurrent technique, metadata of a file identified for truncation ordeletion is updated in a short amount of time such that an internalsnapshot of the file may be deleted at a later time to free storageresources associated with a portion of the identified for deletionduring a truncate operation.

Generally, per-block metadata (also referred to herein as “BMD”) of afile system block stores a total distributed reference count value forthe file system block. Further, a metadata transaction entry created fora file system block that is being deleted indicates return of thereference count value (or “weight”) of the file system block to theper-block metadata of the file system block.

Further, in at least one embodiment of the current technique, a filesystem block associated with a portion of a file being truncated isdeleted based on the ownership status of the file system block. Thus, adelete operation on a file system block decrements the reference countin the per-block metadata of a child block by a full weight or a partialweight depending on whether or not the deleted file system block did notshare the child block or did share the child block.

Further, in at least one embodiment of the current technique, if a filesystem block is not shared but owned by a parent file system block whichpoints to the file system block, the owned file system block is deletedby freeing the file system block. Further, when a file system block of afile of a file system is deleted, metadata such as superblock of thefile, the size of the file, and a mapping pointer in a parent filesystem block pointing to the file system block is updated to indicatethat the file system block is a free file system block which may bereused. It should be noted that any number of improvement may beemployed when deleting a file system block shared between a file and aninternal snapshot copy of the file.

In at least some implementations in accordance with the technique asdescribed herein, the use of the managing truncation of files of filesystems technique can provide one or more of the following advantages:improving memory utilization by reducing the number of times file systemlocks are acquired and released, improving I/O performance of a systemby reducing the number of I/Os generated during a file delete and filetruncate operations, improving host I/O performance by efficientlydeleting or truncating a file by reducing the number of metadatatransactions and the number of times metadata is retrieved from astorage device, improving performance of truncate or delete operationsby efficiently using storage resources (e.g., CPU and cache) of asystem, and efficiently updating metadata entries.

Referring now to FIG. 1, shown is an example of an embodiment of acomputer system that may be used in connection with performing thetechnique or techniques described herein. The computer system 10includes one or more data storage systems 12 connected to host systems14 a-14 n through communication medium 18. The system 10 also includes amanagement system 16 connected to one or more data storage systems 12through communication medium 20. In this embodiment of the computersystem 10, the management system 16, and the N servers or hosts 14 a-14n may access the data storage systems 12, for example, in performinginput/output (I/O) operations, data requests, and other operations. Thecommunication medium 18 may be any one or more of a variety of networksor other type of communication connections as known to those skilled inthe art. Each of the communication mediums 18 and 20 may be a networkconnection, bus, and/or other type of data link, such as hardwire orother connections known in the art. For example, the communicationmedium 18 may be the Internet, an intranet, network or other wireless orother hardwired connection(s) by which the host systems 14 a-14 n mayaccess and communicate with the data storage systems 12, and may alsocommunicate with other components (not shown) that may be included inthe computer system 10. In at least one embodiment, the communicationmedium 20 may be a LAN connection and the communication medium 18 may bean iSCSI or SAN through fibre channel connection.

Each of the host systems 14 a-14 n and the data storage systems 12included in the computer system 10 may be connected to the communicationmedium 18 by any one of a variety of connections as may be provided andsupported in accordance with the type of communication medium 18.Similarly, the management system 16 may be connected to thecommunication medium 20 by any one of variety of connections inaccordance with the type of communication medium 20. The processorsincluded in the host computer systems 14 a-14 n and management system 16may be any one of a variety of proprietary or commercially availablesingle or multiprocessor system, such as an Intel-based processor, orother type of commercially available processor able to support trafficin accordance with each particular embodiment and application.

It should be noted that the particular examples of the hardware andsoftware that may be included in the data storage systems 12 aredescribed herein in more detail, and may vary with each particularembodiment. Each of the host computers 14 a-14 n, the management system16 and data storage systems may all be located at the same physicalsite, or, alternatively, may also be located in different physicallocations. In connection with communication mediums 18 and 20, a varietyof different communication protocols may be used such as SCSI, FibreChannel, iSCSI, FCoE and the like. Some or all of the connections bywhich the hosts, management system, and data storage system may beconnected to their respective communication medium may pass throughother communication devices, such as a Connectrix or other switchingequipment that may exist such as a phone line, a repeater, a multiplexeror even a satellite. In at least one embodiment, the hosts maycommunicate with the data storage systems over an iSCSI or fibre channelconnection and the management system may communicate with the datastorage systems over a separate network connection using TCP/IP. Itshould be noted that although FIG. 1 illustrates communications betweenthe hosts and data storage systems being over a first connection, andcommunications between the management system and the data storagesystems being over a second different connection, an embodiment may alsouse the same connection. The particular type and number of connectionsmay vary in accordance with particulars of each embodiment.

Each of the host computer systems may perform different types of dataoperations in accordance with different types of tasks. In theembodiment of FIG. 1, any one of the host computers 14 a-14 n may issuea data request to the data storage systems 12 to perform a dataoperation. For example, an application executing on one of the hostcomputers 14 a-14 n may perform a read or write operation resulting inone or more data requests to the data storage systems 12.

The management system 16 may be used in connection with management ofthe data storage systems 12. The management system 16 may includehardware and/or software components. The management system 16 mayinclude one or more computer processors connected to one or more I/Odevices such as, for example, a display or other output device, and aninput device such as, for example, a keyboard, mouse, and the like. Adata storage system manager may, for example, view information about acurrent storage volume configuration on a display device of themanagement system 16. The manager may also configure a data storagesystem, for example, by using management software to define a logicalgrouping of logically defined devices, referred to elsewhere herein as astorage group (SG), and restrict access to the logical group.

It should be noted that although element 12 is illustrated as a singledata storage system, such as a single data storage array, element 12 mayalso represent, for example, multiple data storage arrays alone, or incombination with, other data storage devices, systems, appliances,and/or components having suitable connectivity, such as in a SAN, in anembodiment using the techniques herein. It should also be noted that anembodiment may include data storage arrays or other components from oneor more vendors. In subsequent examples illustrated the techniquesherein, reference may be made to a single data storage array by avendor, such as by EMC Corporation of Hopkinton, Mass. However, as willbe appreciated by those skilled in the art, the techniques herein areapplicable for use with other data storage arrays by other vendors andwith other components than as described herein for purposes of example.

An embodiment of the data storage systems 12 may include one or moredata storage systems. Each of the data storage systems may include oneor more data storage devices, such as disks. One or more data storagesystems may be manufactured by one or more different vendors. Each ofthe data storage systems included in 12 may be inter-connected (notshown). Additionally, the data storage systems may also be connected tothe host systems through any one or more communication connections thatmay vary with each particular embodiment and device in accordance withthe different protocols used in a particular embodiment. The type ofcommunication connection used may vary with certain system parametersand requirements, such as those related to bandwidth and throughputrequired in accordance with a rate of I/O requests as may be issued bythe host computer systems, for example, to the data storage systems 12.

It should be noted that each of the data storage systems may operatestand-alone, or may also included as part of a storage area network(SAN) that includes, for example, other components such as other datastorage systems.

Each of the data storage systems of element 12 may include a pluralityof disk devices or volumes. The particular data storage systems andexamples as described herein for purposes of illustration should not beconstrued as a limitation. Other types of commercially available datastorage systems, as well as processors and hardware controlling accessto these particular devices, may also be included in an embodiment.

Servers or host systems, such as 14 a-14 n, provide data and accesscontrol information through channels to the storage systems, and thestorage systems may also provide data to the host systems also throughthe channels. The host systems do not address the disk drives of thestorage systems directly, but rather access to data may be provided toone or more host systems from what the host systems view as a pluralityof logical devices or logical volumes. The logical volumes may or maynot correspond to the actual disk drives. For example, one or morelogical volumes may reside on a single physical disk drive. Data in asingle storage system may be accessed by multiple hosts allowing thehosts to share the data residing therein. A LUN (logical unit number)may be used to refer to one of the foregoing logically defined devicesor volumes. An address map kept by the storage array may associate hostsystem logical address with physical device address.

In such an embodiment in which element 12 of FIG. 1 is implemented usingone or more data storage systems, each of the data storage systems mayinclude code thereon for performing the techniques as described herein.In following paragraphs, reference may be made to a particularembodiment such as, for example, an embodiment in which element 12 ofFIG. 1 includes a single data storage system, multiple data storagesystems, a data storage system having multiple storage processors, andthe like. However, it will be appreciated by those skilled in the artthat this is for purposes of illustration and should not be construed asa limitation of the techniques herein. As will be appreciated by thoseskilled in the art, the data storage system 12 may also include othercomponents than as described for purposes of illustrating the techniquesherein.

The data storage system 12 may include any one or more different typesof disk devices such as, for example, an ATA disk drive, FC disk drive,and the like. Thus, the storage system may be made up of physicaldevices with different physical and performance characteristics (e.g.,types of physical devices, disk speed such as in RPMs), RAID levels andconfigurations, allocation of cache, processors used to service an I/Orequest, and the like.

In certain cases, an enterprise can utilize different types of storagesystems to form a complete data storage environment. In one arrangement,the enterprise can utilize both a block based storage system and a filebased storage hardware, such as a VNX™ or VNXe™ system (produced by EMCCorporation, Hopkinton, Mass.). In such an arrangement, typically thefile based storage hardware operates as a front-end to the block basedstorage system such that the file based storage hardware and the blockbased storage system form a unified storage system.

Referring now to FIG. 2, shown is an example of an embodiment of acomputer system such as a unified data storage system that may be usedin connection with performing the technique or techniques describedherein. As shown, the unified data storage system 10 includes a blockbased storage system 12 and file based storage hardware 34. While theblock based storage system 12 may be configured in a variety of ways, inat least one embodiment, the block based storage system 12 is configuredas a storage area network (SAN), such as a VNX™ or VNXe™ system, asproduced by EMC Corporation of Hopkinton, Mass. While the file basedstorage hardware 34 may be configured in a variety of ways, in at leastone embodiment, the file based storage hardware 34 is configured as anetwork attached storage (NAS) system, such as a file server systemproduced by EMC Corporation of Hopkinton, Mass., configured as a headerto the block based storage system 12.

The computer system 10 includes one or more block based data storagesystems 12 connected to host systems 14 a-14 n through communicationmedium 18. The system 10 also includes a management system 16 connectedto one or more block based data storage systems 12 through communicationmedium 20. In this embodiment of the computer system 10, the managementsystem 16, and the N servers or hosts 14 a-14 n may access the blockbased data storage systems 12, for example, in performing input/output(I/O) operations, data requests, and other operations. The communicationmedium 18 may be any one or more of a variety of networks or other typeof communication connections as known to those skilled in the art. Eachof the communication mediums 18 and 20 may be a network connection, bus,and/or other type of data link, such as a hardwire or other connectionsknown in the art. For example, the communication medium 18 may be theInternet, an intranet, network or other wireless or other hardwiredconnection(s) by which the host systems 14 a-14 n may access andcommunicate with the block based data storage systems 12, and may alsocommunicate with other components (not shown) that may be included inthe computer system 10. In one embodiment, the communication medium 20may be a LAN connection and the communication medium 18 may be an iSCSIor fibre channel connection.

Each of the host systems 14 a-14 n and the block based data storagesystems 12 included in the computer system 10 may be connected to thecommunication medium 18 by any one of a variety of connections as may beprovided and supported in accordance with the type of communicationmedium 18. Similarly, the management system 16 may be connected to thecommunication medium 20 by any one of variety of connections inaccordance with the type of communication medium 20. The processorsincluded in the host computer systems 14 a-14 n and management system 16may be any one of a variety of proprietary or commercially availablesingle or multiprocessor system, such as an Intel-based processor, orother type of commercially available processor able to support trafficin accordance with each particular embodiment and application.

In at least one embodiment of the current technique, block based datastorage system 12 includes multiple storage devices 40, which aretypically hard disk drives, but which may be tape drives, flash memory,flash drives, other solid state drives, or some combination of theabove. In at least one embodiment, the storage devices may be organizedinto multiple shelves 44, each shelf containing multiple devices. In theembodiment illustrated in FIG. 2, block based data storage system 12includes two shelves, Shelf1 44A and Shelf2 44B; Shelf1 44A containseight storage devices, D1-D8, and Shelf2 also contains eight storagedevices, D9-D16.

Block based data storage system 12 may include one or more storageprocessors 46, for handling input/output (I/O) requests and allocations.Each storage processor 46 may communicate with storage devices 40through one or more data buses 48. In at least one embodiment, blockbased data storage system 12 contains two storage processors, SP1 46A,and SP2 46B, and each storage processor 46 has a dedicated data bus 48for each shelf 44. For example, SP1 46A is connected to each storagedevice 40 on Shelf1 44A via a first data bus 48A and to each storagedevice 40 on Shelf2 44B via a second data bus 48B. SP2 46B is connectedto each storage device 40 on Shelf1 44A via a third data bus 48C and toeach storage device 40 on Shelf2 44B via a fourth data bus 48D. In thismanner, each device 40 is configured to be connected to two separatedata buses 48, one to each storage processor 46. For example, storagedevices D1-D8 may be connected to data buses 48A and 48C, while storagedevices D9-D16 may be connected to data buses 48B and 48D. Thus, eachdevice 40 is connected via some data bus to both SP1 46A and SP2 46B.The configuration of block based data storage system 12, as illustratedin FIG. 2, is for illustrative purposes only, and is not considered alimitation of the current technique described herein. Thus, it should benoted that data storage system 12 may include any number of storageprocessors.

In addition to the physical configuration, storage devices 40 may alsobe logically configured. For example, multiple storage devices 40 may beorganized into redundant array of inexpensive disks (RAID) groups.Although RAID groups are composed of multiple storage devices, a RAIDgroup may be conceptually treated as if it were a single storage device.As used herein, the term “storage entity” may refer to either a singlestorage device or a RAID group operating as a single storage device.

Storage entities may be further sub-divided into logical units. A singleRAID group or individual storage device may contain one or more logicalunits. Each logical unit may be further subdivided into portions of alogical unit, referred to as “slices”. In the embodiment illustrated inFIG. 2, storage devices D1-D5, is sub-divided into 3 logical units, LU142A, LU2 42B, and LU3 42C. The LUs 42 may be configured to store a datafile as a set of blocks striped across the LUs 42.

The unified data storage system 10 includes a file based storagehardware 34 that includes at least one data processor 26. The dataprocessor 26, for example, may be a commodity computer. The dataprocessor 26 sends storage access requests through physical data link 36between the data processor 26 and the block based storage system 12. Thedata link 36 may be any one or more of a variety of networks or othertype of communication connections as known to those skilled in the art.The processor included in the data processor 26 may be any one of avariety of proprietary or commercially available single ormultiprocessor system, such as an Intel-based processor, or other typeof commercially available processor able to support traffic inaccordance with each particular embodiment and application. Further,file based storage hardware 34 may further include control station 30and additional data processors (such as data processor 27) sharingstorage device 40. A dual-redundant data link 32 interconnects the dataprocessors 26, 27 to the control station 30. The control station 30monitors a heartbeat signal from each of the data processors 26, 27 inorder to detect a data processor failure. If a failed data processorcannot be successfully re-booted, the control station 30 will “fenceoff” the failed data processor and re-assign or fail-over the dataprocessing responsibilities of the failed data processor to another dataprocessor of the file based storage hardware 34. The control station 30also provides certain server configuration information to the dataprocessors 26, 27. For example, the control station maintains a bootconfiguration file accessed by each data processor 26, 27 when the dataprocessor is reset.

The data processor 26 is configured as one or more computerized devices,such as file servers, that provide end user devices (not shown) withnetworked access (e.g., NFS and CIFS facilities) to storage of the blockbased storage system 12. In at least one embodiment, the control station30 is a computerized device having a controller, such as a memory andone or more processors. The control station 30 is configured to providehardware and file system management, configuration, and maintenancecapabilities to the data storage system 10. The control station 30includes boot strap operating instructions, either as stored on a localstorage device or as part of the controller that, when executed by thecontroller following connection of the data processor 26 to the blockbased storage system 12, causes the control station 30 to detect theautomated nature of a file based storage hardware installation processand access the data processor 26 over a private internal managementnetwork and execute the file based hardware installation process.

Generally, designs of block-based and file-based data storage systemsoften follow parallel paths. Further, many of the features provided byblock-based storage, such as replication, snaps, de-duplication,migration, failover, and non-disruptive upgrade, are similar to featuresprovided for file-based data storage systems. For user convenience,block-based and file-based storage systems are sometimes co-located,essentially side-by-side, to allow processing of both block-based andfile-based host IOs in a single combined system as illustrated in FIG.2. Alternatively, both block-based and file-based functionality may becombined in an unified data path architecture. The unified data patharchitecture brings together IO processing of block-based storagesystems and file-based storage systems by expressing both block-basedobjects and file-based objects in the form of files. These files areparts of an underlying, internal set of file systems, which is stored ona set of storage units served by a storage pool. Because bothblock-based objects and file-based objects are expressed as files, acommon set of services can be applied across block-based and file-basedobjects for numerous operations, such as replication, snaps,de-duplication, migration, failover, non-disruptive upgrade, and/or manyother services, as these services are performed similarly for both blockand file objects on the same underlying type of object—a file. Further,the unified data path architecture increases storage utilization byreallocating storage resources once allocated to block-based storage tofile-based storage, and vice-versa. As block-based objects (e.g., LUNs,block-based vVols, and so forth) and file-based objects (e.g., filesystems, file-based vVols, VMDKs, VHDs, and so forth) are expressed asunderlying files, storage units released by any underlying file or filescan be reused by any other underlying file or files, regardless ofwhether the files represent block-based objects or file-based objects.Additional details regarding the unified data path architecture isdescribed in U.S. patent application Ser. No. 13/828,322 for “UnifiedDataPath Architecture”, filed Mar. 14, 2013, the contents and teachingsof which are hereby incorporated by reference in their entirety.

In at least one embodiment of the current technique, the unified datapath architecture requires a file system to be hosted on a mapped LUN asa file system on a file.

FIG. 3 illustrates one of the many ways of constructing storage extentsfrom a group of physical devices. For example, RAID Group 64 may beformed from physical disk devices 60. The data storage system bestpractices of a policy may specify the particular RAID level andconfiguration for the type of storage extent being formed. The RAIDGroup 64 may provide a number of data storage LUNs 62. An embodiment mayalso utilize one or more additional logical device layers on top of theLUNs 62 to form one or more logical device volumes 61. The particularadditional logical device layers used, if any, may vary with the datastorage system. It should be noted that there may not be a 1-1correspondence between the LUNs of 62 and the volumes of 61. In asimilar manner, device volumes 61 may be formed or configured fromphysical disk devices 60. Device volumes 61, LUNs 62 and physical diskdevices 60 may be configured to store one or more blocks of data or oneor more files organized as a file system. A storage extent may be formedor configured from one or more LUNs 62.

The data storage system 12 may also include one or more mapped devices70-74. A mapped device (e.g., “thin logical unit”, “direct logicalunit”) presents a logical storage space to one or more applicationsrunning on a host where different portions of the logical storage spacemay or may not have corresponding physical storage space associatedtherewith. However, the “thin logical unit” (“TLU”) mapped device is notmapped directly to physical storage space. Instead, portions of themapped storage device for which physical storage space exists are mappedto data devices such as device volumes 61 a-61 b, which are logicaldevices that map logical storage space of the data device to physicalstorage space on the physical devices 60 a-60 b. Thus, an access of thelogical storage space of the “thin logical unit” (“TLU”) mapped deviceresults in either a null pointer (or equivalent) indicating that nocorresponding physical storage space has yet been allocated, or resultsin a reference to a data device which in turn references the underlyingphysical storage space.

Referring to FIG. 4, shown is a logical representation of a LUNpresented to a host and organized as a file system that may be includedin an embodiment using the techniques herein. A user of data storagesystem 12 accesses data from LUNs stored on disk drives 60 in fixedsized chunks. Each fixed size chunk is known as a slice. One or moreslices are grouped together to create a slice pool. Host system 14provisions storage from slice pools for creating LUNs. A LUN 80 isvisible to host system 14 and a user of a data storage system 12.Typically, storage is allocated when host system 14 issues a writerequest and needs a data block to write user's data.

File systems typically include metadata describing attributes of a filesystem and data from a user of the file system. A file system contains arange of file system blocks that store metadata and data. A file systemmapping driver allocates file system blocks from slices of storage forcreating files and storing metadata of a file system. In at least someembodiments of the current technique, the file system block may be 8kilobyte (KB) in size. Further, a user of data storage system 12 createsfiles in a file system. The file system is organized as a hierarchy. Atthe top of the hierarchy is a hierarchy of the directories 82 in thefile system. Inodes of data files 84 depend from the file systemdirectory hierarchy 82. Indirect blocks of data files 86 depend from theinodes of the data files 84. Data block metadata 87 and data blocks ofdata files 88 depend from the inodes of data files 84 and from theindirect blocks of data files 86.

A file system includes one or more file system blocks. Some of the filesystem blocks are data blocks, some file system blocks may be indirectblock, as described above, or some file system blocks are free blocksthat have not yet been allocated to any file in the file system. In anindirect mapping protocol, such as the conventional indirect mappingprotocol of a UNIX-based file system, the indirect mapping protocolpermits any free block of the file system to be allocated to a file ofthe file system and mapped to any logical block of a logical extent ofthe file. This unrestricted mapping ability of the conventional indirectmapping protocol of a UNIX-based file system is a result of the factthat metadata for each file includes a respective pointer to each datablock of the file of the file system, as described below. Each file ofthe file system includes an inode containing attributes of the file anda block pointer array containing pointers to data blocks of the file.There is one inode for each file in the file system. Each inode can beidentified by an inode number. Several inodes may fit into one of thefile system blocks. The inode number can be easily translated into ablock number and an offset of the inode from the start of the block.Each inode of a file contains metadata of the file. Some block pointersof a file point directly at data blocks, other block pointers of thefile points at blocks of more pointers, known as an indirect block.There are at least fifteen block pointer entries in a block pointerarray contained in an inode of a file. The first of up to twelve entriesof block pointers in the inode directly point to the first of up totwelve data blocks of the file. If the file contains more than twelvedata blocks, then the thirteenth entry of the block pointer arraycontains an indirect block pointer pointing to an indirect blockcontaining pointers to one or more additional data blocks. If the filecontains so many data blocks that the indirect block becomes full ofblock pointers, then the fourteenth entry of the block pointer arraycontains a double indirect block pointer to an indirect block thatitself points to an indirect block that points to one or more additionaldata blocks. If the file is so large that the indirect block becomesfull of block pointers and its descendant indirect blocks are also fullof block pointers, then the fifteenth entry of the block pointer arrayincludes another level of indirection where the block pointer entrycontains a triple indirect block pointer to an indirect block thatpoints to an indirect block that points to an indirect block that pointsto one or more additional data blocks. Similarly there exists fourth andfifth level of indirections. Once the indirect blocks at last level ofindirection and its descendant indirect blocks become full of pointers,the file contains a maximum permitted number of data blocks. Further, anindirect block at the last level of indirection is also referred to as aleaf indirect block. However, it should be noted that a file system maybe organized based on any one of the known mapping techniques such as anextent based binary tree mapping mechanism.

Referring to FIG. 5, shown is a representation of a per block metadata(also referred to as “BMD”) for a file system data block that may beincluded in an embodiment using the techniques described herein. Theper-block metadata 75 for a file system data block includes an inodenumber of a file of the file system, the file system data block numberand the logical offset of the file system data block. The per-blockmetadata 75 for a file system data block also includes an internalchecksum protecting the integrity of the information stored in theper-block metadata 75. The per-block metadata for a file system datablock may further include a mapping pointer 76 and a data structureindicating state of the per-block metadata 77. The representation ofper-block metadata 75, as illustrated in FIG. 5, is for illustrativepurposes only, and is not considered a limitation of the currenttechnique described herein.

Referring to FIG. 6, shown is a representation of a mapping pointer 75of a file system data block that may be included in an embodiment usingthe techniques described herein. Each file system data block of a fileis associated with a respective mapping pointer. A mapping pointer of afile system block points to the file system block and includes metadatainformation for the file system block. A file system block associatedwith a mapping pointer may be a data block or an indirect block which inturn points to other data blocks or indirect blocks. A mapping pointerincludes information that help map a logical offset of a file systemblock to a corresponding physical block address of the file systemblock. Mapping pointer 76 includes metadata information such as sharedbit 90, digest bit 91, direct bit 92, virtual bit 93, weight 94, unusedbit 95 and block address 96. Shared bit 90 of mapping pointer 76associated with a file system data block indicates whether the datablock (or data blocks if the mapping pointer is associated with anindirect block) may be shared. Digest bit 91 of mapping pointer 76 for afile system block indicates whether the file system block has beendigested by a deduplication engine. Direct bit 92 of mapping pointer 76for a file system block indicates whether the physical address of thefile system block can be computed algorithmically. Virtual bit 93 ofmapping pointer 76 for a file system block indicates whether the mappingpointer is a virtual pointer. Weight 94 of mapping pointer 76 for a filesystem block indicates a delegated reference count for the mappingpointer 76. The delegated reference count is used by a snapshot copyfacility when a replica of a file is created. Mapping pointers of theinode of the file are copied and included in the inode of the replica ofthe file. In at least one embodiment, mapping pointers of the inode mayinclude mapping pointers pointing to direct data blocks and mappingpointers pointing to indirect blocks. Then, the delegated referencecount values stored in the mapping pointers of the file and the replicaof the file are updated to indicate that the file and the replica of thefile share data blocks of the file. Unused bit 95 of mapping pointer 76for a file system block indicates an unused space reserved for a futureuse. Block address 96 of mapping pointer 76 for a file system blockindicates the block number of the file system block. Alternatively,block address 96 of mapping pointer 76 may indicate a Virtual BlockMetadata (“VBM”) identification number which points to a VBM object thatpoints to a data block and includes metadata for the data block. Thus,VBM Id 96 is used to find an object including virtual block metadata.Thus, a VBM object includes file system data block mapping pointer asdescribed in FIG. 6. It also includes a total distributed weight for theVBM object which is the sum of weights of each mapping pointer for afile system block pointing to the VBM object. The VBM object may furtherincludes a mapping pointer which may point to a file system block oranother VBM object such that the mapping pointer includes thedistributed weight for the mapping pointer.

In response to a request by a client of a storage system to create asnapshot copy of a production file, a virtual block mapping pointer iscreated that provides a mapping information to a logical block storingdata of the file system block of the production file. The file systemblock includes a pointer pointing back to the metadata of the virtualblock mapping pointer. Thus, a new kind of block pointer called virtualblock mapping (VBM) pointer enables a migration or re-organization ofdata blocks to be performed in a non-disruptive fashion that istransparent to a file system manager because pointers to logical datablocks may be changed dynamically without having to change blockpointers in inodes and indirect blocks pointing to the data blocks.

The representation of mapping pointer 76, as illustrated in FIG. 6, isfor illustrative purposes only, and is not considered a limitation ofthe current technique described herein. Thus, it should be noted thatfields 90-95 included in the mapping pointer 76 may reside in differentmetadata structures.

Referring to FIG. 7, shown is a more detailed representation ofcomponents that may be included in an embodiment using the techniquesdescribed herein. As shown in FIG. 7, for example, a production fileinode 100 (also referred to as “working file”) includes a set of mappingpointers representing a file system block hierarchy of the productionfile. The set of mapping pointers includes the first mapping pointerfield which further includes a delegated reference count 112, sharedflag 113 indicating whether the data block pointed to by the firstmapping pointer is shared by other data blocks, and a block pointer 114pointing to a first file system data block (“Data Block 0”) 121. Theblock pointer 114 is a file system block number of the first data block121. The first data block 121 has associated per-block metadata 122including a reference count 123. The per-block metadata 122 of the firstdata block 121, for example, is organized as table separate from thefirst data block 121 and indexed by the block number of the first datablock 121. Further, the set of mapping pointers includes the secondmapping pointer which includes a delegated reference count 115, sharedflag 116, and a block pointer 117 pointing to a second file system datablock (“Data Block 1”) 124. The second data block 124 has associatedper-block metadata 125 including a reference count 126. Further, the setof mapping pointers include a mapping pointer that points to an indirectblock and includes a delegated reference count 118, shared flag 119, anda block pointer 120 pointing to the indirect block 127. The indirectblock (“Indirect block 0”) 124 has associated per-block metadata 140including a reference count 141. The indirect block 124 includes mappingpointers for a set of data blocks pointed to by the indirect block 124,such as, the first mapping pointer which includes a delegated referencecount 128, shared flag 129, and a block pointer 130 pointing to a thirdfile system data block (“Data Block 2”) 134 and the second mappingpointer which includes a delegated reference count 131, shared flag 132,and a block pointer 133 pointing to a fourth file system data block(“Data Block 4”) 135. The third data block 134 has associated per-blockmetadata 136 including a reference count 137 and fourth data block 135has associated per-block metadata 138 including a reference count 139.

In the example of FIG. 7, a delegated reference count such as 112 isassociated with the parent-child block relationship indicated by theblock pointer 114 by storing the delegated reference count in one ormore bytes of a mapping block pointer field. The delegated referencecount 112, however, could be associated with the parent-child blockrelationship in other ways. For example, the delegated reference countcould be stored in a metadata table of the production file inode 100.

In the example of FIG. 7, a delegated reference count such as 112, 115,118, 128, 131 has an initial full-weight value of 1,000, and thereference count such as 123, 126, 141, 137, 139 in the per-blockmetadata such as 122, 125, 140, 136, 138 of file system block such as121, 124, 127, 134, 135 also has an initial full-weight value of 1,000.In other words, the initial full-weight value of 1,000 should beunderstood as representing a full ownership interest (i.e., a 100%ownership interest) of the file system data block. A snapshot copyfacility delegates a partial ownership interest to a snapshot copy whensharing occurs between a snapshot copy and a production file.

As shown in FIG. 8, when the snapshot copy facility creates a firstsnapshot copy of the production file, the snapshot copy facilityallocates an inode 150 for the snapshot copy, and copies the content ofthe production file inode 100 into the snapshot copy inode 150. Then thesnapshot copy facility decrements each of the delegated reference counts112, 115, 118 included in the set of mapping pointers of the productionfile inode 100 by a partial-weight value of 10, and sets the delegatedreference counts 151, 154, 157 in each of the mapping block pointerfields of the snapshot inode 150 to the same partial-weight value of 10.Block pointers 153, 156, 159 in snapshot inode 150 of the snapshot copyof production file now points to the same file system blocks 121, 124,127 and sharing status flag for file system blocks 121, 124, 127 in theproduction file inode 100 and the snapshot copy inode 150 are updated toindicate that file system blocks 121, 124, 127 are shared by theproduction file and the snapshot copy of the production file. Thus, thetotal distributed weight of file system blocks 121, 124, 127 which istotal of the delegated reference counts 112, 115, 118 of the primaryinode 100 and delegated reference counts 151, 154, 157 of the snapshotinode 150 stays same with the value of 1,000.

Although in general a partial-weight value is simply smaller than afull-weight value, in most cases the ratio of the full-weight value tothe partial-weight value may be greater than the maximum number ofsnapshot copies of a production file. For some applications, arelatively small partial weight in relationship to a limited number ofsnapshot copies would also permit identification of child blocksexclusively owned or shared only among snapshot files, permitting arapid delete of all snapshot copies simply by scanning for file systemblocks having a reference count value below a certain threshold, andde-allocating all such blocks.

In general, the delegated reference counting mechanism as shown in FIGS.7-9 results in the reference count in the per-block metadata of a childblock of a file system being equal to the sum of all the delegatedreference counts associated with all of the child's parent blocks in thefile system block hierarchy of the file system. The block sharing causedby creation of snapshot copies does not change the reference count inthe per-block metadata of a child block.

When a snapshot copy of a file is deleted, a portion of the file istruncated, or a portion of a snapshot copy of the file is truncated,each indirect block in a file system block hierarchy corresponding to aportion of a file or a snapshot copy of the file which is being deletedor truncated is evaluated such that a sibling indirect data block isdetermined for each indirect data block from file system hierarchies ofsnapshot copies of the file included in a version set to which the filebelongs such that the indirect data block and the sibling indirect datablock shares the most data blocks compared to other indirect data blocksin the file system hierarchies. Upon finding a sibling indirect datablock for an indirect data block that has been selected for deletion,reference count for each shared data block pointed to by the indirectdata block is returned to corresponding shared data block mappingpointer included in the sibling indirect data block instead of updatingper-block metadata of each shared data block. It should be noted that anindirect data block may be selected from a file system hierarchy of afile when the file is deleted, the file is truncated, zeros are writtento a portion of the file (also referred to as “punching a hole”), ordata blocks are freed and returned to a storage device. Returning weightvalue for each shared data block pointed to by an indirect data block ofa file to reference count values in a sibling indirect data block mayalso be referred to as “reverse indirect block split” operation as itoperates in an opposite manner to a write split operation describedabove herein.

Generally, a delete or truncate operation for a file and/or snap of thefile traverses a file system hierarchy for the file or the snap of thefile in a top-down manner such that each indirect data block included insuch file system hierarchy is traversed until each file system datablock in each leaf indirect data block is evaluated. The processing of aleaf indirect data block includes processing each file system data blockof the leaf indirect block where sharing status of each file system datablock is evaluated. If a file system block of a snap processed fordeletion is no longer referenced by any other active snaps, the filesystem block is deallocated and storage space associated with the filesystem block is freed. However, if a file system block of a snapprocessed for deletion is referenced by other active snaps, the filesystem block is not freed but metadata (e.g., delegated reference count)associated with the file system block is updated to decrement thereference to the file system data block.

Generally, the total distributed reference count value of a file systemdata block and a sharing status is maintained in per-block metadata ofthe file system data block. Typically, per-block metadata of a filesystem data block is either read from a cache or retrieved from astorage device if it does not reside in the cache to evaluate thesharing status of the file system data block. Further, in such a system,the delegated reference count value included in a mapping pointer of afile system data block is compared with the total distributed referencecount value (“weight”) stored in the per-block metadata.

In at least one embodiment of the current technique, sharing status ofeach file system data block listed in the list 242 is evaluated todetermine whether the file system data block can be freed if no otheractive snap refers to the file system data block. Upon determining thata file system block is “owned” indicating that the file system datablock has not been shared by any other active snap and is onlyreferenced by a file selected for deletion, the file system block may bedeallocated and storage space associated with the file system data blockmay be reclaimed as free storage. However, upon determining that a filesystem data block is “shared” indicating that the file system data blockis either shared by more than one snap where one of the snap may nothave been selected for deletion, the reference to the file system datais decremented by using the delegating reference counting mechanismdescribed above herein.

Referring to FIG. 9, shown is a flow diagram illustrating a flow of datain the data storage system. With reference also to FIGS. 1-8, in atleast one embodiment of the current technique, a file delete or a filetruncate operation is performed on a file (step 500). A file deleteoperation deletes the entire contents of a file identified for deletion.However, a file truncate operation performed on a file reduces the sizeof the file by deleting a portion of the file. Upon receiving a requestto either delete a file or truncate a file by deleting a portion of thefile, an internal snapshot copy of the file is created (step 502). Theportion of the file identified for truncation is deleted by usingdelegated reference count mechanism described above herein whichincludes updating metadata of file system blocks associated with theportion of the file (step 504). The file system blocks associated withthe portion of the file identified for truncation are shared with theinternal snapshot copy of the file thereby enabling a storage system todelete such shared file system blocks in a short amount of time bysimply returning respective distributed weight of each shared filesystem block. The internal snapshot copy of the file is asynchronouslydeleted by a background process at a later time (step 506).

While the invention has been disclosed in connection with preferredembodiments shown and described in detail, their modifications andimprovements thereon will become readily apparent to those skilled inthe art. Accordingly, the spirit and scope of the present inventionshould be limited only by the following claims.

What is claimed is:
 1. A method for use in managing truncation of filesof file systems, the method comprising: receiving a request to delete aportion of a file of a file system, wherein the portion of the fileidentified for deletion is associated with a set of indirect datablocks, each indirect data block of the set of indirect blocks pointingto a set of data blocks; creating an internal replica of the file,wherein the internal replica represents a state of the file at aparticular prior point in time, wherein the internal replica shares aset of data blocks of the file with the file; deleting the portion ofthe file by updating metadata of the file enabling the deletion of theportion of the file without having to deallocate each data blockassociated with the portion of the file; asynchronously deleting theinternal replica of the file in background by de-allocating the set offile system data blocks at a later time; and creating another internalreplica of the file upon receiving a subsequent request to deleteanother portion of the file during asynchronous deletion of the internalreplica of the file in the background.
 2. The method of claim 1, whereinthe replica is an internal snapshot of the file, wherein the internalsnapshot is not accessible to a user of the file.
 3. The method of claim1, wherein the portion of the file is deleted based on a delegatedreference counting mechanism.
 4. The method of claim 1, wherein the fileis truncated in size by deleting the portion of the file.
 5. The methodof claim 1, wherein the file includes a set of file system blocks,wherein a mapping pointer is associated with a file system block,wherein the mapping pointer includes a distributed weight indicatingwhether the file system block has been shared.
 6. The method of claim 1,wherein updating the metadata of the file includes adding weight of eachfile system block shared between the replica and the file to a mappingpointer associated with each shared file system block.
 7. The method ofclaim 1, further comprising: receiving a subsequent request to truncatethe file during deletion of the portion of the file; creating anotherreplica of the file; and asynchronously deleting the another replica inbackground.
 8. The method of claim 1, wherein a file system mappingcomponent manages the file system, wherein the file system includes aset of files, each file associated with an inode, wherein the inode of afile includes metadata of the file, wherein the file system isassociated with a set of sparse volumes, wherein a sparse volumeincludes a set of slices, each slice of the set of slices is a logicalrepresentation of a subset of physical disk storage.
 9. The method ofclaim 1, wherein the file system resides on a storage system, whereinthe storage system includes a disk drive system comprising a pluralityof Redundant Array of Inexpensive Disks (RAID) systems, each RAID systemof the plurality of RAID systems having a first disk drive and a seconddisk drive.
 10. A system for use in managing truncation of files of filesystems, the system comprising a processor configured to: receive arequest to delete a portion of a file of a file system, wherein theportion of the file identified for deletion is associated with a set ofindirect data blocks, each indirect data block of the set of indirectblocks pointing to a set of data blocks; create an internal replica ofthe file, wherein the internal replica represents a state of the file ata particular prior point in time, wherein the internal replica shares aset of data blocks of the file with the file; delete the portion of thefile by updating metadata of the file enabling the deletion of theportion of the file without having to deallocate each data blockassociated with the portion of the file; asynchronously delete theinternal replica of the file in background by de-allocating the set ofdata blocks at a later time; and create another internal replica of thefile upon receiving a subsequent request, to delete another portion ofthe file during asynchronous deletion of the internal replica of thefile in the background.
 11. The system of claim 10, wherein the replicais an internal snapshot of the file, wherein the internal snapshot isnot accessible to a user of the file.
 12. The system of claim 10,wherein the portion of the file is deleted based on a delegatedreference counting mechanism.
 13. The system of claim 10, wherein thefile is truncated in size by deleting the portion of the file.
 14. Thesystem of claim 10, wherein the file includes a set of file systemblocks, wherein a mapping pointer is associated with a file systemblock, wherein the mapping pointer includes a distributed weightindicating whether the file system block has been shared.
 15. The systemof claim 10, wherein updating the metadata of the file includes addingweight of each file system block shared between the replica and the fileto a mapping pointer associated with each shared file system block. 16.The system of claim 10, further comprising: receive a subsequent requestto truncate the file during deletion of the portion of the file; createanother replica of the file; and asynchronously delete the anotherreplica in background.
 17. The system of claim 10, wherein a file systemmapping component manages the file system, wherein the file systemincludes a set of files, each file associated with an Mode, wherein theMode of a file includes metadata of the file, wherein the file system isassociated with a set of sparse volumes, wherein a sparse volumeincludes a set of slices, each slice of the set of slices is a logicalrepresentation of a subset of physical disk storage.
 18. The system ofclaim 10, wherein the file system resides on a storage system, whereinthe storage system includes a disk drive system comprising a pluralityof Redundant Array of Inexpensive Disks (RAID) systems, each RAID systemof the plurality of RAID systems having a first disk drive and a seconddisk drive.